Lecture 3 - Early Fishes 1. Early Chordates 2. Conodonts 3. Early Vertebrates 4. Jawless fishes 5. Agnatha/ Gnathostomes junction 6. Placoderms 7. Chondrichthyes
Cephalochordates (lancelets) Early Chordates most primitive chordates? Urochordates (tunicates and salps) - lack vertebrae, brain, image forming eyes, & heart notochord: stiffening rod that provides support crucial to vertebrate embryological development
Cephalochordates are sister to vertebrates embrace your inner lancelet!
Synapomorphies: Cranium present Craniata (= Vertebrates) Cartilage or bone or both are present Heart chambered Neural crests
Conodonts 600 200 mybp elements were abundant in fossil beds Not until 1980 s did we find fossilized soft body parts cartilaginous head skeleton
Agnatha - Jawless Fishes Agnatha appeared 530 mybp previously given superclass status now recognized as paraphyletic Myxinomorphs now considered separate in own superclass still used as informal adjective for jawless fishes
Agnatha Ostracoderms Name means shell-skinned referring to bony shield that covered head and thorax heavily armored first ossified bones evolve jawless & no pelvic fin
Ostracoderms are paraphyletic making ostracoderm a false designation likely 4 superclasses of jawless fishes
Hagfishes - Class Myxini 81 species Live deep scavengers & predators Strictly marine isoosmotic - no osmoregulation
Hagfishes - Class Myxini 4 rudimentary hearts 70-200 pairs of slime glands eye spots lack vertebrae
Rasping tongue
Hagfish feeding
Hagfishes - Class Myxini Reproduction.? cash prize for information on the reproduction of Myxine glutinosa remains unclaimed since 1854 from the Royal Danish Academy of Sciences Development No larval stage, direct development from eggs
Lampreys - Class Petromyzontida ~ 41 species Osmoregulate - found in freshwater and ocean Larval stage called ammocoete Ammocoete undergo metamorphosis Ammocoete filter feeds
Lampreys
Lampreys Reproduction semelparous males build nest eggs hatch after 12-14 days and ammocoete emerges ammocoete burrows into mud or silt in river or stream, with head emerged filter feeding can stay like this for up to seven years Non-parasitic spp. form spp. pairs with parasitic spp.
Agnatha - Gnathostomata Junction Gnathostomes: jawed vertebrates
Agnatha - Gnathostomata Junction Gnathostomes: jawed vertebrates
Agnatha - Gnathostomata Junction Gnathostomes: jawed vertebrates Synapomorphies: Jaws modified from gill arches Paired limbs Vertebral centra usually present
Previously thought that a shark like ancestor gave rise to all jawed vertebrates but this fossil debunks that hypothesis characteristics of both Placoderms and Osteichthyans
Entelognathus primordialis
Entelognathus primordialis,
Placoderms Plate-skinned fresh & salt water arose ~430 mybp disappeared ~350 mybp
- evolved towards reduced armament Synapomorphies: Head and shoulder girdles with dermal bony plates Five gill arches Placoderms
Chondrichthyes - Cartilaginous fishes arose ~420 mybp
Chondrichthyes - Cartilaginous fishes extant cartilaginous fishes
Chondrichthyes - Cartilaginous fishes Synapomorphies: Cartilaginous skeleton, not ossified Skull with no sutures
Chondrichthyes - Cartilaginous fishes Synapomorphies (cont d): Internal fertilization with claspers High blood concentration of urea
Chondrichthyes - Cartilaginous fishes Reproduction oviparous: lay eggs ~40% species viviparous: live birth ~10% species placental attachment ovoviviparous: egg hatches internally and give live birth ~50% species ancestral state
Chondrichthyes - Cartilaginous fishes Split into two subclasses: Elasmobranchii (sharks & rays) Holocephali (ratfishes)
Holocephali whole-head oviparous
Holocephalans - Chimaeras - Ratfish Synapomorphies: Gill cover over 4 gill openings Upper jaw fused to cranium
Holocephalans - Chimaeras - Ratfish Some species possess head claspers - more speciose in the past